Particular electrical properties are of significant advantage in polymer dielectric insulating compounds for use in high voltage service, such as high voltage direct current applications. For instance, the slope of a plotted curve (.alpha.) for the volume resistivity of a given insulating material in relation to temperature changes therein, should be as flat as possible, whereas the slope of a plotted curve (.beta.) for the volume resistivity of the same insulation in relation to the dc stress in the insulation should be as steep as possible for optimum performance. This combination of electrical properties for an insulation, together with a high direct current dielectric strength, provides a superior direct current insulating assuring an optimum dc cable performance for a given delta temperature (.DELTA.T) across the thickness of the body of insulation.